Wood composites such as boards and panels fabricated from a cellulosic material and a cementatious product as the binder are known in the art and are considered desirable in the construction trades since they generally exhibit, in comparison to conventional resin bonded wood panels or boards such as particleboard, improved fire, decay, termite and weathering resistance, and also in comparison to conventional concrete, are of lower density and enjoy a lower thermal conductivity.
Unlike common concrete with rigid aggregates, wood composites employing a cement based binder cannot maintain their shape and dimension after a forming pressure is relieved due to "springback", unless considerable strength in the cement has first been developed.
As wood is an organic material and cement inorganic, they are inherently incompatible, and will not normally bond together unless additional bonding techniques are employed, such as the mineralization of wood and the neutralizing of sugars and other extractives in the wood, or maintaining the compressed cement-wood product in this state for an unacceptable long period of time. As a result, the procedures involved in producing cement-bonded wood composites is significantly different from those used when making concrete products or resin bonded wood products.
Although cement bonded wood composite is, in many respects superior to resin bonded wood products, such as particleboard, the former has, hitherto, only enjoyed limited success in North America which may be attributable to the fact that the fabrication of cement bonded wood composites is not conducive to mass production, as the set and cure rates of the cement is slow, and large plant inventories and associated equipment are necessary to permit the boards or panels to mature to full strength.
Historically, Portland cement was first employed as a cementacious binder for the cellulosic material, with various proposals being advanced in order to expedite the production of this type of product.
Carbon dioxide (CO.sub.2) has long been recognized as a means of accelerating the initial cure of Portland cement by carbonation. For example, K. G. Bierlich, in U.S. Pat. No. 3,468,993 issued Sep. 23, 1969, in context of conventional concrete utilizing Portland cement and solid aggregate, disclosed subjecting this mixture with water to pressure compaction, and during the early stage of hydration and gel formation, exposing the compressed mixture to an atmosphere of carbon dioxide at superatmospheric pressure in order to obtain early strength.
In U.S. Pat. No. 4,746,481 issued May 24, 1988--Schmidt, also having recognized that CO.sub.2 accelerates significantly the carbonization of Portland cement, developed a method of producing a wood based composite bonded with this type of cement. According to this process, using a carbon dioxide injection press, CO.sub.2, through unheated press platens, is injected into a compressed mat of wood material, Portland cement and water, to achieve early strength in the board or panel being produced. No consideration, however, was given to the adverse effect sugar and other wood extractives can have on Portland cement bonding, as discussed below.
Realizing that Portland cement and the amount of cement required in the production of cement bonded wood composites is high, as is its overall production costs, others have attempted to use pulverized water quenched blast-furnace cement, which is also known as latent hydraulic cement or slag cement, as a binder for the fiberous material. Although slow to mature, slag cement exhibits a greater final strength than Portland cement.
Forss in U.S. Pat. No. 4,708,918 issued Nov. 24, 1987 noted that while wood sugars or other extractives in the wood has a deleterious effect on Portland cement as a binder, a pulverized slag cement, as a binder, on the other hand, appears insensitive or immune to these wood sugars and extractives. Accordingly, Forss discloses combining pulverized slag cement (pulverized slag is not hydraulic on its own, in the sense it cannot set in the presence of water alone for a reasonable time period) with an activator having an alkaline reaction and water glass. Since large quantities of activator are used, slag cement-bonded wood composites produced by this prior art technique do develop considerable initial strength in the press, and enables the pressed product to be removed from the press without post-press clamping during the early stage of curing in order to avoid springback. However, this technique still requires an excessive press time (e.g. over 10-15 minutes for a 12 mm thick board), with the press time required dramatically increasing with increasing board thickness. Further, because significant quantities of activator are required, the cost component of the activator is high relative to the remaining raw materials. It has also been observed that the finished products do not display great dimensional stability, and are subject to water staining due to the high alkaline content.
Accordingly, until the present invention, pulverized blast-furnace cement, also known as slag cement, while having inherent material cost saving attributes, has not been effectively employed in the production of wood composites where early strength or set up of the slag binder in a press is a desired object, and which is a prerequisite to using slag cement as a binder in the production of wood composites on a mass production, high output basis.
Wood is the least expensive raw material used in a cementatious wood composite and within limits, its strength increases with increasing wood content. Conversely, however, as the wood content increases, the fire, decay and termite resistance capability of the composite decreases due to the fact that more wood may be exposed for combustion and other adverse environments. Thus, and in keeping with another aspect of this invention, a cementatious wood product can be produced which exhibits greater resistance to fire than its counterparts heretofore produced, and additionally, following a fire, should have greater in situ fire load carrying strength.